Hot embossing of high performance polymers

Hot embossing and injection moulding belong to the established plastic moulding processes in microengineering. Based on experimental and theoretical findings, a variety of microstructures have been replicated using these processes. However, beside the technology also the moulding materials their physical and chemical properties determines the process and their parameters. Especially high temperature semicrystalline polymers like PEEK and LCP are well suited for many applications in microfluidics. The moulding of these polymers by hot embossing requires a precise setting of temperatures for successful moulding. The present publication describes the moulding of high performance polymers by hot embossing and thermoforming. The focus is set to the process parameters and the required heating conditions for the replication of micro- and nanostructured mould inserts. In detail, the process conditions for hot embossing of LCP, PEEK, FEP and PSU will be discussed.     

Hot embossing of thermoplastic multilayered stacks

The combination of different polymer materials during replication offers additional opportunities for fabrication and functionality of microsystems. Different surface and structural properties of polymers allow for improvements in microsystems for example by means of hydrophilic and hydrophobic combinations in microfluidic devices. Due to its high flexibility and precision hot embossing as one of the established micro replication processes facilitates processing of several polymer layers in one single process step. By this multi-component process micro structured systems consisting of thin layers of different polymers with adapted surface properties are fabricated. In this paper we describe the challenge of molding different types of polymers and some applications for multi-component micro systems.     

Hot embossing of transparent high aspect ratio micro parts

Accurate replications of complex, high aspect ratio nano and micro structured parts are still challenging due to the comparatively high surface-volume ratio. The critical process step of automated, undistorted demoulding in high precision replication techniques like hot embossing or thermal nano imprint require good process control at elevated temperatures just below the solidification of the thermoplastic material and higher adhesive forces of the polymer part to the substrate plate than to the structured tool insert. The required increase in interfacial surface to the substrate plate is typically done by a rough substrate plate which results in milky, in-transparent residual layer. We demonstrate a process modification which keeps the advantage of precise automated demoulding and allows for replication of micro structured parts with optically transparent residual layer even for high aspect ratio structures resulting in high demoulding forces.     

Mechanical structuring,surface treatment and tribological characterization of steel mould inserts for micro powder injection moulding

Manufacturing of ceramic and metallic micro components in micro powder injection moulding (μPIM) requires mould inserts offering high wear resistance and a sufficient demoulding behaviour. Within the frame of this research μPIM mould inserts made from low and high alloyed tool steel were structured by micro milling and finished by micro peening and ultrasonic wet peening. Influence of surface condition on wear and demoulding behaviour of the steels in μPIM with ceramic feedstock was characterized using a laboratory tribotester simulating powder injection moulding and a specially adapted static friction tester. Results indicate that performance of mould inserts in micro powder injection moulding depends not only on hardness, surface condition and homogeneity of the mould insert materials but also is strongly influenced by the characteristics of the feedstock, like composition of the binder or amount and hardness of the ceramic particles.     

Polymethylmethacrylate/polyethyleneglycol-based partially water soluble binder system for micro ceramic injection moulding

In micro powder injection moulding polyethylene-wax binder systems have been widely established for many years enabling the fabrication of dense ceramic or metal micro structured parts. With respect to complete organic moiety removal a solvent debinding step prior to thermal decomposition using hexane as organic solvent has to be applied dissolving the wax prior to thermal decomposition of the polyethylene. The development of environmentally friendly binder systems must consider the substitution of any organic solvent or even the solvent pre-debinding. In this work a modified process chain, starting with a reactive feedstock mixture consisting of a thermally curable methylmethacrylate/polymethylmethacrylate resin, low molecular mass polyethyleneglycol and submicron-sized zirconia as ceramic filler, followed by feedstock polymerization at elevated temperature, pelletizing, injection moulding, debinding and sintering, will be presented. Prior to replication important feedstock properties like temperature and solid load dependent melt viscosity as well as the real solid load was measured guaranteeing a successful mould filling. Two different debinding strategies—with and without water-assisted predebinding—were pursued and the resulting sinter part densities as well as surface qualities were compared. Zirconia test specimen parts with a density around 99 % of the theoretical density could be obtained successfully.     

Effect of vacuum venting and mold wettability on the replication of micro-structured surfaces

Micro injection molding enables the manufacture of micro-scale features with good accuracy at high production rates. However, the replication of complex micro and nano features is still challenging hindering the development of new functional surface topographies. The marked thermal gradient between injected polymer and mold surface and the reduced dimensions promote a rapid drop of melt temperature that causes the incomplete filling of the micro features. This study aims to investigate the combined effects of vacuum venting and mold wettability on the replication of micro-structured surfaces. A low-viscosity polystyrene and a cyclic olefin copolymer were selected and their wetting properties were evaluated. The results showed that a polymer with high wetting properties and an elevated viscosity dependence on temperature improves the replication of the micro features. Moreover, high interfacial effects can be exploited to significantly enhance the filling ratio when applying vacuum venting.

     

A novel approach to realize the local precise variotherm process in micro injection molding

Variotherm is special mould temperature controlling concept which realizes the rapid heating/cooling of mould during material processing in order to extend the freezing time of materials. A novel variotherm concept based on silicon wafer combined with micro electric resistance heating structures is purposed and prototyped in this study. The manufacture process of this variotherm unit was introduced and its heating/cooling performance was tested and analyzed by thermal graphic method, which indicate that the new variotherm concept has excellent thermal control efficiency and precise temperature distributions.     

Influence of surface condition on wear behavior of ��PIM mould inserts made of tool steel and cemented carbide

Mould inserts made of low alloyed tool steel and cemented carbide were tested using a laboratory tribometer simulating micro powder injection moulding (μPIM) to characterize the influence of surface condition due to different machining (micro milling or electrical discharge machining) and finishing (ultrasonic wet peening or abrasive micro peening) processes on wear in contact with ceramic feedstocks of different composition. Results show that abrasive micro peening is a suitable finishing process to ensure an excellent wear resistance of EDM processed mould inserts made from ultrafine cemented carbide even in contact with alumina feedstock of high abrasiveness. Ultrasonic wet peening on the other hand allows effective deburring and smoothing of relatively soft steel surfaces after micro milling.     

High aspect ratio micro tool manufacturing for polymer replication using μEDM of silicon, selective etching and electroforming

Mass fabrication of polymer micro components with high aspect ratio micro-structures requires high performance micro tools allowing the use of low cost replication processes such as micro injection moulding. In this regard an innovative process chain, based on a combination of micro electrical discharge machining (μEDM) of a silicon substrate, electroforming and selective etching was used for the manufacturing of a micro tool. The micro tool was employed for polymer replication by means of the injection moulding process.     

Visualizing analysis for weld line forming in micro injection molding by experimental method

In micro injection molding, the melt flow behavior is important for the final product quality. However, the current process monitoring and measurement technology are not adequate enough to provide a direct analysis access. In the presented study, a glass insert mold designed for performing the direct visual analysis for melt flow phenomena in micro injection molding is introduced. The micro tensile specimen with 0.1 × 0.4 mm² (depth × width) cross section dimension is chosen as the objective part. The correlation between processing parameters (injection pressure, injection speed, mold temperature) and flow behavior was investigated and analyzed. The results show that the injection pressure put an obvious effect on the filling speed through micro cavity. Injection speed can influence the filling time dramatically also. Higher mold temperature brings positive influence with the flowing speed, due to the lower viscosity of polymers in higher mold temperature.     

Visibility-based conformal cooling channel generation for rapid tooling

In plastic injection moulding process, cooling channel design is an essential factor that affects the quality of the moulded parts and the productivity of the process. Non-uniform cooling or long cooling cycle time would result if a poorly designed cooling channel is adopted. Due to limitations of traditional machining processes, the cooling channel is usually formed from straight-line drilled holes and only simple shapes are allowed, regardless of the shape complexity of the part being moulded. With the advent of rapid tooling technology, cooling channels in complex shapes can now be possible. However, there are not many design methodologies for supporting this type of cooling channel. In this paper, a methodology called visibility-based cooling channel generation is proposed for automatic preliminary cooling channel design for rapid tooling. The cooling process between a mould surface and a cooling channel is considered analogous to whether they can be visible from each other. Without loss of generality, the mould surface is approximated by a polyhedral terrain and is normally offset. A number of point light sources together that can illuminate the whole polyhedral terrain are assigned to suitable terrain offset vertices. A cooling channel is then generated by connecting all the assigned light sources. When comparing the conventional verification and redesign methods by melt flow analysis, computer-aided design and, a better design of cooling channel for its mould surface results in a short time independent of the experience of mould engineer.     

Micro assembly injection moulding for the generation of hybrid microstructures

Based on the micro injection moulding process, which has been investigated at the IKV for several years, a process called micro assembly injection moulding is developed and investigated. It combines the joining of hybrid elements with the generation of functional structures. Micro assembly injection moulding is a quite diverse process with many influencing parameters. For basic studies of this process, a special mould technology with maximum flexibility and precision is used. The mould concept includes a special system for the positioning of inlay parts and complex sensor equipment (pressure, temperature endoscopic devices) for controlling and analysing the process. The processing aspects which have been investigated are the production of movable microstructures by using incompatible polymers, the generation of fluidic hollow structures by lost core technology and the overmoulding of wires and optical fibres. In all cases, the choice of the different materials, the temperature layout of the process and the mechanical strain of inlay parts play a significant role. Furthermore, the precision in positioning of inlay parts and the demoulding of hybrid structures are of great importance.The investigations are part of the SFB (Sonderforschungsbereich) 440 Assembly of Hybrid Micro Systems and were financially supported by the DFG (Deutsche Forschungsgemeinschaft).This paper was presented at the Conference of Micro System Technologies 2001 in March 2001.     

Requirements of an industrially applicable microcutting process for steel micro-structures

 Micro-cutting offers good potentialities in order to manufacture small and medium lot sizes of micro-parts with arbitrary geometry at an economically reasonable expense. Either by direct machining or as a means to fabricate moulds for micro injection moulding, the major advantages turn out to be large removal rates, good compliance with tolerance ranges, high surface quality and a wide choice of materials which can be processed. Particularly if highly wear resistant materials are to be processes, as it is the case in mould fabrication for powder injection moulding, micro cutting of steel is a very eligible option. Consequently, the possibility to manufacture wear resistant micro structures of high aspect ratios by mechanical cutting is demonstrated with regard to its specific requirements in terms of transferability of the laboratory process into an industrial manufacturing process. Accordingly the paper focuses on repeatability of machining results and machining capabilities. Received: 10 August 2001/Accepted: 24 September 2001     

Hot embossing of micro and sub-micro structured inserts for polymer replication

Today replication of microstructured parts is state of the art in laboratory and commercial use. Beside the process of injection molding hot embossing enables the accurate replication of polymer structures in a broad variety of thermoplastic polymers even in the nanometer range. Characteristic for the most replication processes dealing with thermoplastic polymers is the use of microstructured mold inserts based on metals. In this paper we describe an alternative to the established mold inserts––the use of so called interstage mold inserts. These interstage mold inserts are replicated in high performance polymers and technical thermoplastics and can be fabricated many times by a previous replication step from a master even in the sub-micro range. Aspects like suitable material combinations, demolding behaviour, long time stability, production rate, and the quality of structures will be discussed. Because of the high flexibility the process of hot embossing is used for the fabrication of the microstructured interstage mold inserts and their replications.     

Factors affecting the replication quality of micro metal gears produced by μ-PIM

μ-PIM is a promising technology for micro-manufacture, by which complex structured micro components can be produced economically. In this paper, iron micro gear arrays with different diameters were produced by μ-PIM. The influence of die temperature (T _m), the injection pressure (P _i) and the injection speed (V _i) on the replication quality has been studied. The results showed that the replication quality was mainly affected by the T _m. Micro gears with best shape retention were obtained when T _m was set to be 80°C, P _i for 100 MPa and V _i for 60 cm³/s. The green compacts were debinded and subsequently sintered. Micro gears with good shape retention and 0.0053 wt. % residual carbon were obtained after debinding and sintering process. The micro hardness turned out 8 GPa measured by a nano-mechanical test instrument.     

Modeling of multi-connected porous passageway for mould cooling

Cooling channel design in the plastic injection moulding process is of paramount importance to the performance of the mould, influencing the quality of the parts being produced and productivity of the process. However, cooling channel design is usually limited to relatively simple configurations as well as conventional machining processes, such as straight-line drilling, and milling, etc. The cooling performance may not meet the expectations of the mould engineers.This paper proposes an alternative design method for a conformal cooling passageway with multi-connected porous characteristics based on the duality principle. The proposed method can provide a more uniform cooling performance between the mould plate and the conformal cooling passageway than the existing conformal cooling channel design. Injection mould defects like warpage or hot spots can be avoided.In this study, a 3D mould plate model was offset negatively and the location of the proposed multi-connected porous cooling passageway was identified. The negatively offset model was decomposed into a finite number of cubical cells via the sub-boundary spatial enumerated cell decomposition. Then a duality relationship between the primal and the dual graphs was developed. This provided the preliminary layout of the multi-connected porous passageway for the coolant flow in multiple directions. The cooling channel axis design of the multi-connected porous passageway, illustrated by the skeleton from the dual graph, was created. Following a Boolean difference operation, the proposed multi-connected porous cooling passageway inside the mould plate was able to be generated and fabricated with the aid of rapid tooling technologies. A real-life case study for the design of a multi-connected porous cooling passageway was implemented and examined. The effects of coolant flow and cooling performances, analyzed by computational fluid dynamics simulation, were validated.     

Micro assembly injection moulding with plasma treated inserts

Injection moulding is one of the most common manufacturing processes for thermoplastics. By injection moulding, polymer micro components can be produced in large numbers at low costs. Micro assembly injection moulding as a combination of multi-component injection moulding and micro injection moulding enables to avoid offline process steps, as joining is already accomplished in the mould by overmoulding. In case of joint hybrid micro structures, the bonding strength between two components affects the stability of the micro system and is thus important for the part quality. Investigations carried out at the Institute of Plastics Processing (IKV), Aachen, Germany, deal with methods to increase the bonding strength of a plastic and a non-plastic part joint. As shown in the following, a plasma treatment of the inserts is an appropriate approach for this aim.     

Replication of large scale micro pillar array with different diameters by micro injection molding

The capillary force is always used as the driving force of microfluidic chips. In this study, the capillary force of blood smart diagnostic microfluidic chip which fabricated by micro-injection molding (μ-IM) is offered by the structure of micro pillar array. And the detection effect of blood smart diagnostic microfluidic chips is affected by the replication and height distribution of large scale micro pillar array. So the effect of process parameters on the micro-structure and the height distribution of micro pillar is studied. The mold design is also an important factor affecting micro parts properties. In this study, a steel mold insert with almost 15,500 micro blind cavities was fabricated by milling, electrical discharge machine and Femtosecond Laser process. Polymethyl methacrylate -Polystyrene copolymer (SMMA NAS 30) was used as the molding material. The single factor trail and orthogonal experiment approach were adopted to investigate the effect of several process parameters and the significant effect factors affecting the replication of micro pillar. And the height distribution of micro pillar array was investigated by scanning electron microscope (SEM) and universal tool-measuring microscope to measure the replication quality. The results reveal that the replication of micro pillar is sensitive to the flow direction of the polymer melt. The height of micro pillar increases with the increase of mold temperature and injection speed. Moreover, the height distribution of micro pillar along and against flow direction was tightly related to the thermomechanical history of material during the molding process.

     

Plastic injection mould cooling system design by the configuration space method

The cooling system of an injection mould is very important to the productivity of the injection moulding process and the quality of the moulded part. Despite the various research efforts that have been directed towards the analysis, optimization, and fabrication of cooling systems, support for the layout design of the cooling system has not been well developed. In the layout design phase, a major concern is the feasibility of building the cooling system inside the mould insert without interfering with the other mould components. This paper reports a configuration space (C-space) method to address this important issue. While a high-dimensional C-space is generally required to deal with a complex system such as a cooling system, the special characteristics of cooling system design are exploited in the present study, and special techniques that allow C-space computation and storage in three-dimensional or lower dimension are developed. This new method is an improvement on the heuristic method developed previously by the authors, because the C-space representation enables an automatic layout design system to conduct a more systematic search among all of the feasible designs. A simple genetic algorithm is implemented and integrated with the C-space representation to automatically generate candidate layout designs. Design examples generated by the genetic algorithm are given to demonstrate the feasibility of the method.     

Conformal bubbler cooling for molds by metal deposition process

Molds with conformal bubbler cooling channels have been developed in order to reduce the cycle time of the plastic injection process, which in turn has increased the production rate. In addition, part warpage can be greatly reduced in the injection process because the temperature distribution is uniform over the mold surfaces. Molds with conformal cooling designs, however, are still limited to computer simulations for generating the cooling channels or calculating the cooling rates [1], [2], [3], [4]. This is mainly because there is “virtually” no fabrication technique that can effectively make the complicated conformal cooling channels. Even though metal deposition processes have the potential to create such complex mold shapes, most of the metal deposition processes are still in the developmental stage in laboratories. Molds formed by some of the metal deposition processes are not suitable for real industry use because the costs are still too expensive. This paper describes how to create a mold with conformal bubbler cooling tunnels through a metal deposition process that can actually be used in plastic injection industries. Experiments are conducted to confirm the normal temperature distribution over the mold surfaces.